How Deep Impact Works

Comets are traveling balls of astronomic history. Their origins go back to the formation of the solar system, approximately 4.6 billion years ago. When the sun was formed, it caused gases and dust to be dispelled into space. Some of these materials later formed planets, while quantities of these gases and dust settled into orbits around but far from the sun.

Comets are thought to be consolidated balls of these materials, containing ice, dust, organic matter and possibly rock, formed approximately 4 billion years ago. As they travel through the solar system, they pick up additional debris. In this way, comets are windows into the history of the solar system. But with diameters of up to 60 miles (100 km), you can't just reach up and snag one in a big net in order to study it.

Still, scientists are finding a way to get at the information: On January 12, 2005, NASA's Discovery Mission Deep Impact launched with the intent to probe beneath the surface of a comet. On July 4, 2005, Deep Impact encountered Comet Tempel 1.

The Basics

Comet Tempel 1 was in its most solid stage, consisting of a nucleus approximate 3.7 miles (6 km) in diameter, when it encountered the Deep Impact spacecraft in July 2005. (For information on comets, including their structure and composition, check out How Comets Work.) The primary goal behind the Deep Impact mission was to study the interior and the exterior of the same comet.

The Deep Impact spacecraft consisted of two parts: a flyby and an impactor. When the spacecraft came close to the comet, the two parts separated. The impactor put itself in the comet's path, causing a collision between the two bodies.

The impact created a crater in the comet that went well below the surface and exposed the protected material below -- the "pristine material" that was formed during the birth of the solar system. By studying both the material that came out of the crater upon impact and the characteristics of the comet that the crater exposed, scientists now have an unprecedented view of the solar system in its infancy. To learn more about impact craters, see Deep Impact: Cratering.

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This animation shows Deep Impact's journey to Comet Tempel 1, including the separation of the impactor from the spacecraft and the way the impactor targets its path to the comet. Click here to view.

The Science Behind the Mission

When scientists were developing the Deep Impact mission, they set forth the following objectives:

Observe how the crater forms

Measure the crater's depth and diameter

Measure the composition of the interior of the crater and the material that is ejected upon its creation

Determine the changes in natural outgassing produced by the impact

They hope that the information they gather from these objectives will help them answer three primary questions about comets:

Where is the pristine material in comets?

Do comets lose their ice or seal it in?

What do we know about crater formation?

Scientists believe the nucleus of a comet consists of two layers: an external layer called the mantle and an internal layer considered to be pristine. As a comet moves through the solar system, its mantle changes. As it approaches the sun, some of the external ice sublimates and is dispelled. It may also encounter and pick up additional debris. The protected, pristine interior of the comet, however, is thought to be unaffected by the comet's travels and could be as it was when the comet was formed. Scientists believe that a study of the differences between the two layers will tell them a great deal about the nature of the solar system, both its formation and its evolution through the years.

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This is a computer-generated model of what Deep Impact's imaging system should see during its encounter with Comet Tempel 1. Click here to view.

Another major question scientists have about comets is whether or not they go dormant or extinct due to the heat of the sun. A dormant comet is one in which the mantle has sealed off the pristine interior layer, and no gases pass from this interior layer to the exterior layer and out of the comet. An extinct comet has no more gases in its nucleus at all, and as such will never change. Results from the Deep Impact mission will give scientists a better view of the nature of the mantle and enable them to determine if Tempel 1 is active, dormant or extinct.

The results of the impactor's collision will provide lots of information about the nature of comets. The formation of the crater, how fast it formed and its final dimensions tell scientists how porous the mantle and the pristine layers are. A study of how the material ejected from the crater site will show both its porosity and density and potentially the mass of the comet as well. Information from the entire cratering process may give some indication of what kind of material actually makes up the comet, which will help scientists understand how the comet formed and how it has evolved over time.

The Muscle and Mind Behind the Mission

The Deep Impact spacecraft consisted of two parts, the flyby spacecraft and the impactor, and was about the size of a sport utility vehicle. The flyby carries a High Resolution Instrument (HRI) and a Medium Resolution Instrument (MRI) for imaging, infrared spectroscopy and optical navigation. It uses a fixed solar array and an NiH2 battery to power itself. The impactor remained attached to the flyby until 24 hours before it impacted Tempel 1.

Once released, the impactor guided itself into the path of the comet using a high-precision star-tracker (which navigates by looking at the stars), the Impactor Target Sensor (ITS) and auto-navigation algorithms specially developed for this mission. The impactor also contained a small hydrazine propulsion system for more precise trajectory and attitude control. The HRI, MRI and ITS worked together to guide the flyby spacecraft to the comet and record scientific data before, during and after the impact.

The complete flight system was launched as a payload on a Boeing Delta II rocket (see How Rocket Engines Work) in January 2005. It encountered Tempel 1 in early July 2005. Twenty-four hours before impact, the impactor detached itself from the flyby spacecraft. At this point, the flyby slowed down and positioned itself to observe the impact as it passes by the comet.

Once the impactor left the flyby spacecraft, it positioned itself to impact the comet on the sunlit side, allowing for better-quality images.

The flyby's imaging equipment observed the nucleus for more than 10 minutes after the impact, imaging the impact, the crater development and the crater interior. The flyby also acquired spectrometry of the nucleus and the crater site. It sent all of the images and spectrometry back to the Deep Space Network on the ground.

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This animation shows Deep Impact's orbital path and a side view showing how the flyby spacecraft releases the impactor into the path of the comet. Click here to view.

How Deep Impact Came About

Deep Impact began when Alan Delamere and Mike Belton were working on a collaboration to study Comet Halley. "We got Halley data and investigated it and found the comet was far blacker than we had imagined, blacker than coal. So we asked ourselves: How could this happen?" Delamere said. "We became increasingly curious as to just how this black layer accumulated." In 1996, Belton and Delamere, now joined by Mike A'Hearn, submitted a proposal to NASA. They wanted to explore another comet, this time a dead one named Phaethon. They had decided to use an impactor to hit the comet and then observe the results. But NASA was not convinced they could hit the comet. NASA wasn't even convinced that Phaethon was a comet.

Delamere, Belton, and A'Hearn continued to think about the project and try to figure out better ways to do it. In 1998, A'Hearn had taken over leadership of the team, and they made a second proposal. This time, they were going to impact an active comet, Tempel 1. They had also added a guidance system to the impactor, increasing the odds that they would be able to control the spacecraft well enough to hit their target. NASA accepted the new proposal and agreed to fund the project. The Deep Impact mission was born.

Deep Impact is a partnership between the University of Maryland, The California Institute of Technology's Jet Propulsion Laboratory and the Ball Aerospace and Technology Corporation.

For more information about Deep Impact and related topics, check out the links on the next page.